Reflection phenomena occur when the impedance of the junction, or transition, between a coaxial plug and a stripline according to the above is incorrect. In order for the junction to have correct impedance, it is necessary for the junction to exhibit at each cross-section thereof the same characteristic impedance as that of coaxial conductor and the stripline with its earthing plane. An impedance-correct junction is difficult to achieve, however, for reasons of a practical and mechanical nature.
When the length of the junction is considerably shorther than the wavelength of the transmitted signal, it is possible to use a junction with incorrect impedance without reflection becoming unacceptably high. In this regard the length of the junction should be at most 5-10% of the wavelength of the signal, which in the case of a 30 GHz-signal, the wavelength of which on a substrate having a high dielectric constant is about 3 mm, means that the junction should have at maximum a length of some tenths of a millimeter. The greatest problem encountered in the construction of a junction according to the foregoing is one of providing a short earthing plane junction in relation to wave-length, i.e. a short connection between the outer casing of the coaxial conductor and the earthing plane associated with the stripline.
One conceivable method is to solder or glue metal foil onto the earthing plane (surface) and onto a part of the earthing wire of the coaxial plug, the earthing wire being placed around (although not in contact with) the central pin on the transverse wall of the plug. This method, however, is unsuitable when the stripline is located within a hermetically sealed apparatus box together with non-encapsulated chips, since glue gives off gases which are liable to damage the chips, and since soldering gives rise to problems related to flux residues. Consequently, glue should not be used at all, and welding, when carried out, should be done prior to mounting the chips. In certain contexts, inter alia when the electrical equipment is to be used in satellites or space vehicles, manufacturing methods and materials, e.g. synthetic resins, which are liable to produce deleterious gases are sometimes not allowed at all.
The conventional method of obtaining an earthing plane junction would seem to be one which involves the use of the metallic connection extending between the earthing plane of the stripline and the bottom of the apparatus box. This results, however, in long and unsafe earthing plane connections which have incorrect impendance. In addition, problems are caused by the mutually different coefficients of linear expansion of a hard substrate, e.g. a ceramic, and the apparatus box, causing the substrate to crack readily. A soft substrate, e.g. teflon, is more durable than a hard substrate, but cannot be used in hermetically sealed apparatus boxes due to the aforesaid generation of gases.